It is desirable for a tire to exhibit good traction characteristics on wet and dry pavements, and for the tire to provide good treadwear and low rolling resistance. In order to reduce the rolling resistance of a tire, rubbers having a high rebound can be utilized in making the tires"" tread. Tires made with such rubbers undergo less energy loss during rolling. The traditional problem associated with this approach is that the tire""s wet traction and wet skid resistance characteristics are compromised. This is because good rolling resistance which favors low energy loss and good traction characteristics which favor high energy loss are viscoelastically inconsistent properties.
In order to balance these two viscoelastically inconsistent properties, mixtures of various types of synthetic and natural rubber are normally utilized in tire treads. For instance, various mixtures of styrene-butadiene rubber (SBR), polybutadiene rubber, and natural rubber are commonly used in automobile tire treads formulations. Styrene-butadiene rubber is included in tire tread formulations primarily to improve the traction characteristics of the tire without greatly compromising tread-wear or rolling resistance.
The versatility of solution SBR (SSBR) synthesis relative to the synthesis of emulsion (ESBR), including control of molecular weight, macrostructure, microstructure, and functionalization, is well established (see Hirao, A.; Hayashi, M. Acta. Polym. 1999, 50, 219-231, and references cited therein). Performance advantages arising from this versatility have led to an acceleration of the replacement of emulsion SBR in the tire industry, and an expansion in the market for random, low vinyl SBR for use in tire compounds (see Autcher, J. F.; Schellenberg, T.; Naoko, T. xe2x80x9cStyrene-Butadiene Elastomers (SBR),xe2x80x9d Chemical Economics Handbook, SRI-International, November, 1997). These developments have stimulated interest in developing technology for commercial production of random, low vinyl solution SBR.
Although anionic initiated synthesis of random medium vinyl solution SBR and random high vinyl solution SBR is easily accomplished by the addition of Lewis bases, these polar modifiers promote randomization at the expense of increased vinyl content (see Antkowiak, T. A.; Oberster, A. E.; Halasa, A. F.; Tate, D. P. J. Polym. Sci., Part A-1, 1972, 10, 1319). Due to the large differences in monomer reactivity ratios of butadiene and styrene, measures must be taken to promote random incorporation of styrene into low vinyl solution SBR. In the absence of such measures, the polymerization leads to a tapered block copolymer with inferior elastomeric performance characteristics (see U.S. Pat. No. 3,558,575).
British Patent 994,726 reports that it is possible to produce random solution SBR by manipulating monomer polymerization rates via control of monomer concentrations throughout the polymerization process without the use of polar modifiers. For solution SBR, this requires that the polymerization proceed in a styrene rich medium throughout the polymerization. In continuous polymerizations the issues associated with maintaining constant monomer concentration ratios while increasing conversion become quite complex.
U.S. Pat. No. 3,787,377 reports that alkali metal alkoxides (NaOR) can be used as polar modifiers in the copolymerization of styrene and butadiene to randomize styrene incorporation without significantly increasing the vinyl content of the rubber. However, alkali metal alkoxide modifiers are so effective that they may actually increase the rate of polymerization of styrene to the extent that it is depleted before the polymerization is complete (see Hsieh, H. L.; Wofford, C. F. J. Polym. Sci., Part A-1, 1969, 7, 461-469). Furthermore, there is typically some undesired increase in vinyl content over what would be expected from an unmodified polymerization (see Hsieh, H. L.; Wofford, C. F. J. Polym. Sci., Part A-1, 1969, 7, 449-460).
A method to prevent the formation of tapered block solution SBR in unmodified polymerizations using standard continuous stirred tank reactors (CSTRs) has been developed. This method involves charging all of the styrene and part of the 1,3-butadiene being polymerized into a first polymerization zone. The first polymerization zone is typically a continuous stirred tank reactor. The amount of styrene charged into the first polymerization zone will typically be at least 5 percent more than the amount of styrene bound into the styrene-butadiene rubber being synthesized. It is important for a conversion within the range of about 60 percent to about 90 percent to be attained in the first polymerization zone. Additional 1,3-butadiene monomer is charged into a second polymerization zone, such as a second continuous stirred tank reactor. Typically from about 20 percent to about 40 percent of the total amount 1,3-butadiene charged will be charged into the second polymerization zone. It is also important for a 1,3-butadiene conversion of at least about 90 percent to be attained in the second polymerization zone and for the total conversion (styrene and 1,3-butadiene) to be limited to a maximum of about 95 percent in the second polymerization zone.
This invention more specifically discloses a process of synthesizing random styrene-butadiene rubber having a low level of branching and a low vinyl content which comprises: (1) continuously charging 1,3-butadiene, styrene, an alkyl lithium initiator, and an organic solvent into a first polymerization zone, (2) allowing the 1,3-butadiene and styrene to copolymerize in the first polymerization zone to total conversion which is within the range of about 60 percent to about 90 percent to produce a polymer cement containing living styrene-butadiene chains, (3) continuously charging the polymer cement containing living styrene-butadiene chains and additional 1,3-butadiene monomer into a second polymerization zone, wherein from 20 percent to 40 percent of the total amount of 1,3-butadiene changed is charged into the second polymerization zone, (4) allowing the copolymerization to continue in the second polymerization zone to a conversion of the 1,3-butadiene monomer of at least 90 percent, wherein the total conversion of styrene and 1,3-butadiene in the second polymerization zone is limited to a maximum of 95 percent, (5) withdrawing a polymer cement of random styrene-butadiene rubber having living chain ends from the second reaction zone, (6) killing the living chain ends on the random styrene-butadiene rubber, and (7) recovering the random styrene-butadiene rubber from the polymer cement, wherein the copolymerizations in the first polymerization zone and the second polymerization zone are carried out at a temperature which is within the range of about 70xc2x0 C. to about 100xc2x0 C., and wherein the amount of styrene charged into the first polymerization zone is at least 5 percent more than the total amount of styrene bound into the random styrene-butadiene rubber. The living chain ends on the random styrene-butadiene rubber can optionally be killed by the addition of a coupling agent, such as tin tetrachloride.
The present invention also reveals a cement of living styrene-butadiene rubber which is comprised of an organic solvent and polymer chains that are derived from 1,3-butadiene and styrene, wherein the polymer chains are terminated with lithium end groups, wherein the polymer chains have a vinyl content of less than 10 percent, wherein less than 5 percent of the total quantity of repeat units derived from styrene in the polymer chains are in blocks containing five or more styrene repeat units, and wherein the molar amount of polar modifier in the cement of the living styrene-butadiene rubber is at a level of less than 20 percent of the number of moles of lithium end groups on the polymer chains of the living styrene-butadiene rubber. Such cements of living styrene-butadiene rubber made by the process of this invention can be easily coupled because they contain very low levels of polar modifiers.
The polymerizations of the present invention are carried out continuously in a first polymerization zone, such as a first reactor, and a second polymerization zone, such as a second reactor. These copolymerizations of 1,3-butadiene and styrene are carried out in a hydrocarbon solvent which can be one or more aromatic, paraffinic or cycloparaffinic compounds. These solvents will normally contain from 4 to 10 carbon atoms per molecule and will be liquid under the conditions of the polymerization. Some representative examples of suitable organic solvents include pentane, isooctane, cyclohexane, methylcyclohexane, isohexane, n-heptane, n-octane, n-hexane, benzene, toluene, xylene, ethylbenzene, diethylbenzene, isobutylbenzene, petroleum ether, kerosene, petroleum spirits, petroleum naphtha and the like, alone or in admixture.
In the solution polymerizations of this invention, there will normally be from 5 to 30 weight percent monomers in the polymerization medium. Such polymerization media are, of course, comprised of the organic solvent, monomers, and an initiator. In most cases, it will be preferred for the polymerization medium to contain from 10 to 25 weight percent monomers. It is generally more preferred for the polymerization medium to contain 15 to 20 weight percent monomers.
In the polymerizations of this invention the styrene, 1,3-butadiene, solvent, and initiator are continuously charged into the first polymerization zone. All of the styrene and a portion of the 1,3-butadiene is charged into the first polymerization zone. The amount of styrene charged into the first polymerization zone is at least 5 percent more than the total amount of styrene bound into the random styrene-butadiene rubber being synthesized. In other words, at least 5 percent more styrene is charged into the first polymerization zone than will be polymerized during the polymerization in the first polymerization and second polymerization zone. It is preferred for the amount of styrene charged into the first polymerization zone to be at least 7 percent more than the total amount of styrene bound into the random styrene-butadiene rubber being synthesized. It is more preferred for the amount of styrene charged into the first polymerization zone to be at least 10 percent more than the total amount of styrene bound into the random styrene-butadiene rubber being synthesized.
The conversion attained in the first polymerization zone will be within the range of about 60 percent to about 90 percent. It is preferred for the conversion attained in the first polymerization zone will be within the range of about 75 percent to about 85 percent. The polymer cement containing living styrene-butadiene chains and additional 1,3-butadiene monomer made in the first polymerization zone is continuously charged into a second polymerization zone. About 20 percent to 40 percent of the total amount of 1,3-butadiene changed into the first polymerization zone and the second polymerization zone is charged into the second polymerization zone. Preferably from 25 percent to 35 percent of the total amount of 1,3-butadiene changed into the first polymerization zone and the second polymerization zone is charged into the second polymerization zone. Most preferably from 27 percent to 33 percent of the total amount of 1,3-butadiene changed into the first polymerization zone and the second polymerization zone is charged into the second polymerization zone.
It is critical for the total conversion (styrene and 1,3-butadiene) attained in the second polymerization zone to be held below about 95 percent and preferably below about 93 percent. However, the 1,3-butadiene will be polymerized in the second reaction zone to a conversion of at least about 90 percent. The 1,3-butadiene will preferably be polymerized in the second reaction zone to a conversion of at least about 95 percent and will most preferably be polymerized to a conversion of 98 percent.
The copolymerizations of styrene and butadiene in the first polymerization zone and the second polymerization zone will be maintained at a temperature which is within the range of about 70xc2x0 C. to about 100xc2x0 C. At temperatures below about 70xc2x0 C. the polymerization is too slow to be commercially acceptable. On the other hand, at temperatures above 100xc2x0 C. thermal induced branching occurs to the extent that it adversely affects the hysteretic properties of the styrene-butadiene rubber. For these reasons, the polymerization temperature will normally be maintained within the range of 75xc2x0 C. to 85xc2x0 C., and will preferably be maintained within the range of 80xc2x0 C. to 90xc2x0 C.
The styrene-butadiene rubber made utilizing the technique of this invention is comprised of repeat units which are derived from 1,3-butadiene and styrene. These styrene-butadiene rubbers will typically contain from about 5 weight percent to about 50 weight percent styrene and from about 50 weight percent to about 95 weight percent 1,3-butadiene. The styrene-butadiene rubber will more typically contain from about 10 weight percent to about 30 weight percent styrene and from about 70 weight percent to about 90 weight percent 1,3-butadiene. The styrene-butadiene rubber will preferably contain from about 15 weight percent to about 25 weight percent styrene and from about 75 weight percent to about 85 weight percent 1,3-butadiene.
In the styrene-butadiene rubbers of this invention, the distribution of repeat units derived from styrene and butadiene is essentially random. The term xe2x80x9crandomxe2x80x9d as used herein means that less than 5 percent of the total quantity of repeat units derived from styrene are in blocks containing five or more styrene repeat units. In other words, more than 95 percent of the repeat units derived from styrene are in blocks containing less than five repeat units. A large quantity of repeat units derived from styrene will be in blocks containing only one styrene repeat unit. Such blocks containing one styrene repeat unit are bound on both sides by repeat units which are derived from 1,3-butadiene.
In styrene-butadiene rubbers containing less than about 30 weight percent bound styrene which are made with the catalyst system of this invention, less than 2 percent of the total quantity of repeat units derived from styrene are in blocks containing five or more styrene repeat units. In other words, more than 98 percent of the repeat units derived from styrene are in blocks containing less than five repeat units. In such styrene-butadiene rubbers, over 40 percent of repeat units derived from styrene will be in blocks containing only one styrene repeat unit, over 75 percent of the repeat units derived from styrene will be in blocks containing less than 3 repeat units and over 95 percent of the repeat units derived from styrene will be in blocks containing less than 4 repeat units. Normally less than 2 percent of the bound styrene in the styrene-butadiene rubber is in blocks of greater than 3 repeat units. Preferably less than 1 percent of the bound styrene in the styrene-butadiene rubber is in blocks of greater than 3 repeat units.
In styrene-butadiene rubbers containing less than about 20 weight percent bound styrene which are made with the catalyst system of this invention, less than 1 percent of the total quantity of repeat units derived from styrene are in blocks containing 4 or more styrene repeat units. In other words, more than 99 percent of the repeat units derived from styrene are in blocks containing less than 4 repeat units. In such styrene-butadiene rubbers, over 60 percent of repeat units derived from styrene will be in blocks containing only one styrene repeat unit and over 95 percent of the repeat units derived from styrene will be in blocks containing less than 3 repeat units. Normally less than 2 percent of the bound styrene in the styrene-butadiene rubber is in blocks of greater than 3 repeat units. Preferably less than 1 percent of the bound styrene in the styrene-butadiene rubber is in blocks of greater than 3 repeat units.
The styrene-butadiene copolymers of this invention also have a consistent composition throughout their polymer chains. In other words, the styrene content of the polymer will be the same from the beginning to the end of the polymer chain. No segments of at least 100 repeat units within the polymer will have a styrene content which differs from the total styrene content of the polymer by more than 10 percent. Such styrene-butadiene copolymers will typically contain no segments having a length of at least 100 repeat units which have a styrene content which differs from the total styrene content of the polymer by more than about 5 percent.
The polymerizations of this invention are initiated by adding an organolithium compound to the first polymerization zone containing the styrene and 1,3-butadiene monomers. The organolithium compounds which can be employed in the process of this invention include the monofunctional and multifunctional initiator types known for polymerizing the conjugated diolefin monomers. The multifunctional organolithium initiators can be either specific organolithium compounds or can be multifunctional types which are not necessarily specific compounds but rather represent reproducible compositions of regulable functionality.
The choice of initiator can be governed by the degree of branching and the degree of elasticity desired for the polymer, the nature of the feedstock and the like. With regard to the feedstock employed as the source of conjugated diene, for example, the multifunctional initiator types generally are preferred when a low concentration diene stream is at least a portion of the feedstock, since some components present in the unpurified low concentration diene stream may tend to react with carbon lithium bonds to deactivate the activity of the organolithium compound, thus necessitating the presence of sufficient lithium functionality so as to override such effects.
The multifunctional organolithium compounds which can be used include those prepared by reacting an organomonolithium compounded with a multivinylphosphine or with a multivinylsilane, such a reaction preferably being conducted in an inert diluent such as a hydrocarbon or a mixture of a hydrocarbon and a polar organic compound. The reaction between the multivinylsilane or multivinylphosphine and the organomonolithium compound can result in a precipitate which can be solubilized, if desired, by adding a solubilizing monomer such as a conjugated diene or monovinyl aromatic compound, after reaction of the primary components. Alternatively, the reaction can be conducted in the presence of a minor amount of the solubilizing monomer. The relative amounts of the organomonolithium compound and the multivinylsilane or the multivinylphosphine preferably should be in the range of about 0.33 to 4 moles of organomonolithium compound per mole of vinyl groups present in the multivinylsilane or multivinylphosphine employed. It should be noted that such multifunctional initiators are commonly used as mixtures of compounds rather than as specific individual compounds. Exemplary organomonolithium compounds include ethyl lithium, isopropyl lithium, n-butyllithium, sec-butyllithium, tert-octyl lithium, n-eicosyl lithium, phenyl lithium, 2-naphthyllithium, 4-butylphenyllithium, 4-tolyllithium, 4-phenylbutyllithium, cyclohexyl lithium and the like.
Exemplary multivinylsilane compounds include tetravinylsilane, methyltrivinylsilane, diethyldivinylsilane, di-n-dodecyldivinylsilane, cyclohexyltrivinylsilane, phenyltrivinylsilane, benzyltrivinylsilane, (3-ethylcyclohexyl) (3-n-butylphenyl)divinylsilane and the like.
Exemplary multivinylphosphine compounds include trivinylphosphine, methyldivinylphosphine, dodecyldivinylphosphine, phenyldivinylphosphine, cyclooctyldivinylphosphine and the like.
Other multifunctional polymerization initiators can be prepared by utilizing an organomonolithium compound, further together with a multivinylaromatic compound and either a conjugated diene or monovinylaromatic compound or both. These ingredients can be charged initially, usually in the presence of a hydrocarbon or a mixture of a hydrocarbon and a polar organic compound as a diluent. Alternatively, a multifunctional polymerization initiator can be prepared in a two-step process by reacting the organomonolithium compound with a conjugated diene or monovinyl aromatic compound additive and then adding the multivinyl aromatic compound. Any of the conjugated dienes or monovinyl aromatic compounds described can be employed. The ratio of conjugated diene or monovinyl aromatic compound additive employed preferably should be in the range of about 2 to 15 moles of polymerizable compound per mole of organolithium compound. The amount of multivinylaromatic compound employed preferably should be in the range of about 0.05 to 2 moles per mole of organomonolithium compound.
Exemplary multivinyl aromatic compounds include 1,2-divinylbenzene, 1,3-divinylbenzene, 1,4-divinylbenzene, 1,2,4-trivinylbenzene, 1,3-divinylnaphthalene, 1,8-divinylnaphthalene, 1,3,5-trivinylnaphthalene, 2,4-divinylbiphenyl, 3,5,4xe2x80x2-trivinylbiphenyl, m-diisopropenyl benzene, p-diisopropenyl benzene, 1,3-divinyl-4,5,8-tributylnaphthalene and the like. Divinyl aromatic hydrocarbons containing up to 18 carbon atoms per molecule are preferred, particularly divinylbenzene as either the ortho, meta or para isomer, and commercial divinylbenzene, which is a mixture of the three isomers, and other compounds such as the ethyl styrenes, also is quite satisfactory.
Other types of multifunctional lithium compounds can be employed such as those prepared by contacting a sec- or tert-organomonolithium compound with 1,3-butadiene, at a ratio of about 2 to 4 moles of the organomonolithium compound per mole of the 1,3-butadiene, in the absence of added polar material in this instance, with the contacting preferably being conducted in an inert hydrocarbon diluent, though contacting without the diluent can be employed, if desired.
Alternatively, specific organolithium compounds can be employed as initiators, if desired, in the preparation of polymers in accordance with the present invention. These can be represented by R(Li)x wherein R represents a hydrocarbyl radical containing from 1 to 20 carbon atoms, and wherein x is an integer of 1 to 4. Exemplary organolithium compounds are methyl lithium, isopropyl lithium, n-butyllithium, sec-butyllithium, tert-octyl lithium, n-decyl lithium, phenyl lithium, 1-naphthyllithium, 4-butylphenyllithium, p-tolyl lithium, 4-phenylbutyllithium, cyclohexyl lithium, 4-butylcyclohexyllithium, 4-cyclohexylbutyllithium, dilithiomethane, 1,4-dilithiobutane, 1,10-dilithiodecane, 1,20-dilithioeicosane, 1,4-dilithiocyclohexane, 1,4-dilithio-2-butane, 1,8-dilithio-3-decene, 1,2-dilithio-1,8-diphenyloctane, 1,4-dilithiobenzene, 1,4-dilithionaphthalene, 9,10-dilithioanthracene, 1,2-dilithio-1,2-diphenylethane, 1,3,5-trilithiopentane, 1,5,15-trilithioeicosane, 1,3,5-trilithiocyclohexane, 1,3,5,8-tetralithiodecane, 1,5,10,20-tetralithioeicosane, 1,2,4,6-tetralithiocyclohexane, 4,4xe2x80x2-dilithiobiphenyl and the like. Some highly preferred functionalized organolithium initiators are N-lithiopiperidine and 3-pyrrolidine-1-propyllithium.
The organolithium compound will normally be present in the polymerization medium in an amount which is within the range of about 0.01 to 1 phm (parts by 100 parts by weight of monomer). In most cases, from 0.01 phm to 0.1 phm of the organolithium compound will be utilized with it being preferred to utilize from 0.025 phm to 0.07 phm of the organolithium compound in the polymerization medium.
Polar modifiers can be used to modify the microstructure of the rubbery polymer being synthesized. However, the amount of polar modifier employed should be limited to keep the vinyl content of the styrene-butadiene rubber being synthesized at a low level. Ethers and amines which act as Lewis bases are representative examples of polar modifiers that can be utilized. Some specific examples of typical polar modifiers include diethyl ether, di-n-propyl ether, diisopropyl ether, di-n-butyl ether, tetrahydrofuran, dioxane, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, triethylene glycol dimethyl ether, trimethylamine, triethylamine, N,N,Nxe2x80x2,Nxe2x80x2-tetramethylethylenediamine (TMEDA), N-methyl morpholine, N-ethyl morpholine, N-phenyl morpholine and the like. Dipiperidinoethane, dipyrrolidinoethane, tetramethylethylene diamine, diethylene glycol, dimethyl ether, TMEDA, tetrahydrofuran, piperidine, pyridine and hexamethylimine are representative of highly preferred modifiers. U.S. Pat. No. 4,022,959 describes the use of ethers and tertiary amines as polar modifiers in greater detail.
After the polymerization has reached the desired level of conversion it is terminated using a standard technique. The polymerization can be terminated with a conventional noncoupling type of terminator (such as, water, an acid and/or a lower alcohol) or with a coupling agent.
Coupling agents can be used in order to improve the cold flow characteristics of the rubber and rolling resistance of tires made therefrom. It also leads to better processability and other beneficial properties. A wide variety of compounds suitable for such purposes can be employed. Some representative examples of suitable coupling agents include: multivinylaromatic compounds, multiepoxides, multiisocyanates, multiimines, multialdehydes, multiketones, multihalides, multianhydrides, multiesters which are the esters of polyalcohols with monocarboxylic acids, and the diesters which are esters of monohydric alcohols with dicarboxylic acids and the like.
Examples of suitable multivinylaromatic compounds include divinylbenzene, 1,2,4-trivinylbenzene, 1,3-divinylnaphthalene, 1,8-divinylnaphthalene, 1,3,5-trivinylnaphthalene, 2,4-divinylbiphenyl and the like. The divinylaromatic hydrocarbons are preferred, particularly divinylbenzene in either its ortho, meta or para isomer. Commercial divinylbenzene which is a mixture of the three isomers and other compounds is quite satisfactory.
While any multiepoxide can be used, liquids are preferred since they are more readily handled and form a relatively small nucleus for the radial polymer. Especially preferred among the multiepoxides are the epoxidized hydrocarbon polymers such as epoxidized liquid polybutadienes and the epoxidized vegetable oils such as epoxidized soybean oil and epoxidized linseed oil. Other epoxy compounds, such as 1,2,5,6,9,10-triepoxydecane, also can be used.
Examples of suitable multiisocyanates include benzene-1,2,4-triisocyanate, naphthalene-1,2,5,7-tetraisocyanate and the like. Especially suitable is a commercially available product known as PAPI-1, a polyarylpolyisocyanate having an average of three isocyanate groups per molecule and an average molecular weight of about 380. Such a compound can be visualized as a series of isocyanate-substituted benzene rings joined through methylene linkages.
The multiimines, which are also known as multiaziridinyl compounds, preferably are those containing three or more aziridine rings per molecule. Examples of such compounds include the triaziridinyl phosphine oxides or sulfides such as tri(1-ariridinyl)phosphine oxide, tri(2-methyl-1-ariridinyl)phosphine oxide, tri(2-ethyl-3-decyl-1-ariridinyl)phosphine sulfide and the like.
The multialdehydes are represented by compounds such as 1,4,7-naphthalene tricarboxyaldehyde, 1,7,9-anthracene tricarboxyaldehyde, 1,1,5-pentane tricarboxyaldehyde and similar multialdehyde containing aliphatic and aromatic compounds. The multiketones can be represented by compounds such as 1,4,9,10-anthraceneterone, 2,3-diacetonylcyclohexanone and the like. Examples of the multianhydrides include pyromellitic dianhydride, styrene-maleic anhydride copolymers and the like. Examples of the multiesters include diethyladipate, triethyl citrate, 1,3,5-tricarbethoxybenzene and the like.
The preferred multihalides are silicon tetrahalides (such as silicon tetrachloride, silicon tetrabromide and silicon tetraiodide) and the trihalosilanes (such as trifluorosilane, trichlorosilane, trichloroethylsilane, tribromobenzylsilane and the like). Also preferred are the multihalogen-substituted hydrocarbons (such as, 1,3,5-tri(bromomethyl)benzene and 2,4,6,9-tetrachloro-3,7-decadiene) in which the halogen is attached to a carbon atom which is alpha to an activating group such as an ether linkage, a carbonyl group or a carbon-to-carbon double bond. Substituents inert with respect to lithium atoms in the terminally reactive polymer can also be present in the active halogen-containing compounds. Alternatively, other suitable reactive groups different from the halogen as described above can be present.
Examples of compounds containing more than one type of functional group include 1,3-dichloro-2-propanone, 2,2-dibromo-3-decanone, 3,5,5-trifluoro-4-octanone, 2,4-dibromo-3-pentanone, 1,2,4,5-diepoxy-3-pentanone, 1,2,4,5-diepoxy-3-hexanone, 1,2,11,12-diepoxy-8-pentadecanone, 1,3,18,19-diepoxy-7,14-eicosanedione and the like.
In addition to the silicon multihalides as described hereinabove, other metal multihalides, particularly those of tin, lead or germanium, also can be readily employed as coupling and branching agents. Difunctional counterparts of these agents also can be employed, whereby a linear polymer rather than a branched polymer results. Monofunctional counterparts can be used to end cap the rubbery polymer. For instance, trialkyl tin chlorides, such as tri-isobutyl tin chloride, can be utilized to end cap the rubbery polymer.
Broadly, and exemplary, in the case of tetrafunctional coupling agents, such as tin tetrachloride, a range of about 0.01 to 1 moles of coupling agent are employed per mole of lithium in the initiator. To attain a maximum level of coupling, it is preferred to utilize about 0.1 to about 2.5 moles of the coupling agent per mole of lithium in the initiator. The larger quantities tend to result in production of polymers containing terminally reactive groups or insufficient coupling. The coupling agent can be added in hydrocarbon solution (e.g., in cyclohexane) to the polymerization admixture in the final reactor with suitable mixing for distribution and reaction.
After the copolymerization has been completed, the styrene-butadiene elastomer can be recovered from the organic solvent. The styrene-butadiene rubber can be recovered from the organic solvent and residue by means such as decantation, filtration, centrification and the like. It is often desirable to precipitate the segmented polymer from the organic solvent by the addition of lower alcohols containing from about 1 to about 4 carbon atoms to the polymer solution. Suitable lower alcohols for precipitation of the segmented polymer from the polymer cement include methanol, ethanol, isopropyl alcohol, normal-propyl alcohol and t-butyl alcohol. The utilization of lower alcohols to precipitate the rubber from the polymer cement also xe2x80x9ckillsxe2x80x9d the living polymer by inactivating lithium end groups. After the segmented polymer is recovered from the solution, steam-stripping can be employed to reduce the level of volatile organic compounds in the rubber.